Studies of mutation by radiation in bacteria are proposed. A specific mutagenic process producing GC to AT transitions will be investigated preferentially. This process will be displayed in the production of two types of mutant bacteria, by suppressor mutation and by conversion mutation. Consideration of the genetic structures suggests that suppressor mutation and conversion mutation result from the same transition, but that the C of the target base-pair is in the transcribed strand of DNA in one case and in the non-transcribed strand of DNA in the other. Comparative studies of these two mutation examples will be developed to confirm and characterize strand-asymmetric DNA repair involved with cellular mutagenesis. The role of inducible SOS repair will be considered, and also the induction of repair by non-dimer photolesions. The proposed procedures will employ nonsense defective bacteriophage to analyze revertant populations so that specific types of reversion may be followed at all times.